The present invention relates to a fluidized catalytic cracking process wherein fresh hydrocarbon charge is converted into lower boiling cracked hydrocarbons, in a reaction zone, wherein cracked hydrocarbon vapors from the reaction zone are separated, in a fractionation zone, into desired fractions including a gas fraction, a naphtha fraction, a light cycle gas oil fraction, an intermediate cycle gas oil fraction and a heavy cycle gas oil fraction. More particularly, the present invention relates to an improved process for obtaining a light cycle gas oil fraction from said product fractionation zone, stripping heavy naphtha components from said intermediate cycle gas oil fraction and recycling said heavy naphtha components to the reaction zone at an elevated temperature for conversion into light naphtha components having an increased octane value.
Commonly in commercial processes for fluidized catalytic cracking of hydrocarbons, a fresh hydrocarbon charge stock is contacted with hot regenerated catalyst in a reaction zone for conversion of said charge stock into lower boiling cracked hydrocarbons and coke. Subsequently, cracked hydrocarbon vapors and spent catalyst are separated, in a reaction vessel, forming a dense phase fluidized bed of spent cracking catalyst and a dilute phase comprising a minor amount of catalyst suspended in said cracked hydrocarbon vapors, the dense phase fluidized catalyst bed has unvaporized hydrocarbons occluded therein. A substantial proportion of these unvaporized hydrocarbons may be volatilized by contacting the dense phase fluidized catalyst bed with a stripping vapor. Commonly, a vapor, such as steam, is introduced into the lower portion of the reaction vessel for maintaining the dense phase catalyst bed in a fluidized state, and for stripping a portion of the occluded hydrocarbons from the spent catalyst. Vapors other than steam may be used for this purpose, for example, the cracked hydrocarbon vapors entering the reactor vessel; flue gas; etc. Unvaporized hydrocarbons remaining upon the spent catalyst is referred to as "coke". From the reactor vessel dense phase fluidized catalyst bed, spent catalyst is transferred from the reaction zone to a regeneration zone for burning coke and regenerating said catalyst.
Cracked hydrocarbon vapors from the reaction zone are transferred into a product fractionation zone wherein the cracked hydrocarbon vapors are separated into desired product fractions. Such product fractions may include a gas fraction, a naphtha fraction, a light cycle gas oil fraction, an intermediate gas oil fraction, and a heavy cycle gas oil fraction. The gas fraction comprises low molecular weight hydrocarbons and hydrogen. The naphtha fraction comprises hydrocarbons boiling in the range of about C.sub.4 -430.degree. F. which are useful in the manufacture of gasoline motor fuels. The light cycle gas oil fraction, having a boiling range of about 400.degree.-650.degree. F., is useful in the manufacture of fuel oils. The intermediate cycle gas oil is a distillate fraction boiling in the range of about 540.degree.-750.degree. F., and although this intermediate cycle gas oil fraction may be used for production of heavy fuel oil, it is common practice for a substantial portion to be recycled to the reaction zone for conversion into additonal lower boiling hydrocarbons. The heavy cycle gas oil fraction comprises a nondistillate bottoms fraction from the fractionation zone.
For economical operation of a fluidized catalytic cracking unit within a petroleum refinery it is desirable to have the ability to adjust the ratio of the various product fractions, according to product demand. For example, in the Winter season the demand for fuel oil increases and it is desirable to include within the light cycle gas oil fraction the maximum amount of hydrocarbons boiling within the fuel oil range. In Summer, demand for gasoline increases and it is desirable to include within the naphtha fraction the maximum amount of gasoline boiling range hydrocarbons. For obtaining flexibility in product distribution, it is common practice to provide steam strippers for adjusting the initial boiling points of the light cycle gas-oil fraction and the intermediate cycle gas oil fraction. Light cycle gas oil from the fractionation zone is charged to the light cycle gas oil stripper and stripping steam is passed therethrough in order to vaporize naphtha boiling range components from the light cycle gas oil fraction. By adjusting the amount of stripping steam employed, more or less of the naphtha hydrocarbons may be vaporized and returned to the fractionation zone for recovery with the naphtha fraction. That is, by controlling the steam stripping of light cycle gas-oil, the ratio of naphtha to light cycle gas oil may be adjusted over a limited range. The naphtha boiling range components stripped from the light cycle gas fraction boil generally in the range of about 400.degree.-430.degree. F., and have a lower octane value than lower boiling naphtha recovered from the product fractionation zone.
Also, it is common practice to provide a steam stripper for the intermediate cycle gas oil fraction such that light cycle gas-oil boiling range components of this fraction may be vaporized and returned to the product fractionation zone for recovery with the light cycle gas-oil fraction. Thus, a portion of light cycle gas-oil boiling range hydrocarbons present in the intermediate cycle gas oil fraction may selectively be recovered as light cycle gas-oil product, or may be included with the intermediate cycle gas-oil for recycle to the reaction zone or use as heavy fuel oil.
One disadvantage of such steam stripping for the adjustment of the initial boiling points of the light cycle gas oil fraction and the intermediate cycle gas oil fraction is, substantial quantities of steam, in the range of about 1 to 10 pounds of steam per barrel of hydrocarbon, are required. This steam passes overhead from the product fractionation zone and is condensed, forming a foul water component containing such impurities as hydrogen sulfide, cyanides, phenols, etc., which must be subjected to substantial treating prior to disposal.